The present invention relates to a plasma processing apparatus that micromachines a sample such as a wafer in a semiconductor manufacturing process, and more particularly to a temperature control apparatus for a sample stage that holds and fastens a semiconductor wafer.
With a trend of semiconductor device miniaturization, processing precision required for etching a sample is increasing year by year. In order to perform high-precision processing on a fine pattern in a wafer surface by the plasma processing apparatus, it is important to control the wafer surface temperature during etching. In order to meet further increased shape precision in recent years, a temperature control technique is required to rapidly control the wafer temperature to be uniform in-plane depending on an etching step during processing.
In order to control the wafer surface temperature in the plasma processing apparatus, the surface temperature of the sample stage contacting the wafer back surface with a heat transfer medium therebetween may be controlled. A sample stage in the prior art is configured such that a refrigerant passage is formed therein and a liquid refrigerant is introduced into the passage, thereby controlling the temperature of the sample stage surface.
This liquid refrigerant is regulated to a target temperature by a cooling system or a heating system in a refrigerant supply apparatus and then is supplied into the sample stage passage. Such a refrigerant supply apparatus is structured such that the liquid refrigerant is temporarily stored in a tank and undergoes temperature regulation before being discharged. Since the liquid refrigerant itself has a large heat capacity, the refrigerant supply apparatus is effective in maintaining constant wafer surface temperature.
However, the refrigerant supply apparatus has a slow temperature response, a difficulty in rapid temperature control, and a low heat exchange efficiency. Therefore, it is difficult to optimally control the wafer surface temperature as the etching progresses.
In light of this, there has conventionally been proposed a cooling apparatus using a so-called direct expansion system that expands a refrigerant in a refrigerant passage (hereinafter referred to as a direct expansion cooling apparatus) which includes a refrigerant circulating system including a compressor for highly pressurizing a refrigerant, a condenser for condensing the highly pressurized refrigerant, and an expansion valve for expanding the refrigerant, all of which are installed in a sample stage and the refrigerant is evaporated in the refrigerant passage of the sample stage for cooling. The direct expansion technique has advantages: the use of refrigerant evaporation latent heat provides high cooling efficiency and the refrigerant evaporation temperature can be controlled rapidly by pressure.
For example, JP-A-2009-272535 (corresponding to US. Patent Publication No. 2009/0277883A1) discloses a technique for allowing the sample stage to act as an evaporator of a refrigerating cycle and constitute a direct expansion cooling system including a compressor, a condenser, and an expansion valve. The prior art also discloses a method in which the dryness degree of the refrigerant discharged from the sample stage is controlled to prevent the refrigerant dryout (disappearance of a liquid film) from occurring in the refrigerant passage and the sample stage is regulated to an in-plane uniform temperature.
In addition, JP-A-H3-31640 discloses a technique in which an air conditioning apparatus using the direct expansion cooling system allows a desired temperature to be achieved by detecting the temperature to be controlled and controlling the heat exchange amount of the condenser. Specifically, the prior art discloses a technique in which a measured room temperature value is compared with a set room temperature value; and if the measured room temperature value is higher than the set room temperature value, the rotational speed of an outdoor fan is increased to increase the heat exchange amount by an outdoor heat exchanger, thereby maintaining the room temperature at the set temperature.
As described above, the cooling principle of the direct expansion cooling system and refrigerating apparatus uses latent heat released when the refrigerant is evaporated from a liquid to a gas; and the expansion valve opening degree in the cycle can be regulated thereby to regulate the refrigerant pressure and to regulate the evaporation temperature. Note that the refrigerant that absorbs heat from an object to be cooled is increased in enthalpy (amount of heat per 1 kg of refrigerant), undergoes adiabatic compression in the compressor, and the heat is exhausted by the condenser.
At this time, the refrigerant temperature at the sample stage position is changed by an exhaust heat amount of the condenser. In other words, it is also important to appropriately control the exhaust heat capacity of the condenser for temperature control of the sample stage. Meanwhile, JP-A-H3-31640 discloses that in the direct expansion cooling system, the refrigerant temperature control in an evaporator can be controlled not only by the expansion valve opening degree but also by the condenser heat exchange amount.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.